The present invention relates to methods of and apparatus for avoiding or minimising unwanted visible line structures in the electronic reproduction of images, by means of a monochromatic light source whose intensity is controlled by an image signal and in whose beam path is arranged an acousto-optic modulator (AOM) cell which is arranged to carry out the reproduction of the image line by line on a reproducing surface, one line in the reproduction consisting of a multiple line constituted by a plurality of sub-lines. Hereinafter such a method and apparatus will be referred to as "of the kind described".
In the case of image reproduction such as in television and in the transmission of press photographs, weather charts, written documents and the like by land-line or radio, the image is scanned and reproduced again line by line. When reproducing the image, use is made of an energy beam which is deflected in lines and which is modulated in accordance with an image signal obtained by scanning the image. This method of reproduction has long been familiar, for example in television engineering or from German Offenlegungschrift No. 22 10 310, but it has the disadvantage that the line structure resulting from the gaussian energy distribution in the energy beam employed and from the spacing between the individual lines becomes visible. These two factors mean that a choice has to be made between two evils.
On the one hand adjoining lines can be spaced further apart than the mean beam width, in which case it is true that the image is of optimum sharpness because of the good physical separation between the image information, but the line brightness, fluctuating as it does in sequence with the lines, becomes disturbingly apparent and is therefore an unwanted line structure. If on the other hand the spacing between the lines is smaller than the mean beam width and the fluctuations in line brightness are eliminated by the resultant overlap, then there is a loss of sharpness. These problems are familiar and are discussed for example on page 590 of the "Elektrotechnische Zeitschift" volume 13, number 22 for Oct. 30, 1961 and in U.S. patent specification No. 3,997,722. To avoid these faults, either the trace spot is unsharpened to a greater or lesser degree to widen it, as a result of which the image becomes less sharp, or else the trace beam is wobbled at high speed transversely to the direction of reproduction. This is admittedly better in that sharpness is preserved but at the junction between one line and the next there is still a residual brightness fault if the amplitude of the wobble is not exactly matched to the line spacing. Slight alterations in the frequency of wobble or mechanical changes in operation mean that the intensity distribution in the direction of wobble alters, in which case the beam is of greater intensity in one direction of deflection than in the other, which once again produces a line structure.
Another possible way of suppressing the unwanted line structure in an image has also been disclosed in GB patent specification No. 1,468,742. When a light beam is used for reproduction, the beam is broken down into a plurality of sub-beams by means of a plurality of apertures situated next to one another in a mask and the sub-beams are then traced out by means of an optical system as a plurality of individual parallel lines, which does away with the gaps between the lines. This method has the disadvantage that light losses occur due to the intervals between the apertures and due to the peripheral areas. Further light losses are caused by the fact that if the sub-beams are all to be of the same intensity, the mask can only be arranged in the central region of the beam profile, where intensity is approximately uniform. Also, such masks are expensive to manufacture and difficulties may occur as a result of dirtying of the masks.
It is an object of the invention to provide a better and more reliable solution to this problem and to establish a method and apparatus by means of which the unwanted line structure can be completely suppressed or minimised without any loss of sharpness occurring. A further problem which exists with image reproduction of this kind is that of adjusting the line width. This may be required if it is desired to scan and reproduce with different resolutions or if a change of scale is to be made between the original and the reproduction. In the case of the GB patent specification No. 1,468,742 mentioned above, the line width can for example be varied only by the width of one mask segment, which means that the line width can only be varied in relatively coarse steps, unless the mask or the objective lens were to be changed for each resolution, i.e. line width, which would not however overcome the disadvantages of the mask arrangement. It is therefore a further object of the invention to make it possible for the line width to be continuously varied without a line-structure arising.